Method of seamless roaming between wireless local area networks and cellular carrier networks

ABSTRACT

A scheme that enables seamless roaming between the WLAN and the cellular carrier network by enabling a user that originates a call in the WLAN and happens to go outside the range of the WLAN to automatically switch over to the cellular carrier network without losing connection with the other party. This solution assumes that the mobile device has the capability to operate in at least two modes that include the WLAN mode and one of the cellular carrier modes, such as the GSM, IS-95 CDMA, IS-136 TDMA, and iDEN.

RELATED APPLICATIONS

[0001] This application claims the benefit of provisional applicationserial No. 60/419,674 filed on Oct. 18, 2002 which is incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

[0002] The proliferation of the wireless local area networks (WLANs) hasled to the search for ways in which its utilization can be increased. ByWLAN, we include all instantiations of such technologies as 802.11a,802.11b, 802.11g, Bluetoooth and any similar WLAN versions. For example,the specification for the IEEE 802.11 version of WLAN includes an accessscheme called the distributed control function, which permits thenetwork to support both data and voice applications. Today voice overwireless local area network (VoWLAN) is a reality. The voice may beencoded and transmitted using voice over internet protocol (VoIP) formatand protocols such as G.711, G.726, G.729, SIP, MEGACO, H.323, or othersimilar protocols that are being developed.

[0003] One of the issues in WLAN is that there is a limited range ofoperation due to power requirements. For example, a typical IEEE 802.11WLAN has a range of at most 300 yards from the access point thatconnects the mobile devices to the wired LAN. Thus, when an IEEE802.11-based mobile device roams beyond this range, any call in progressis forcibly terminated.

SUMMARY OF THE INVENTION

[0004] The present invention concerns a scheme that enables seamlessroaming between the WLAN and the cellular carrier network. The methodenables a user that originates a call in the WLAN and happens to gooutside the range of the WLAN to automatically switch over to thecellular carrier network without losing connection with the other party.This solution assumes that the mobile device has the capability tooperate in at least two modes that include the WLAN mode and one of thecellular carrier modes, such as the GSM, IS-95 CDMA, IS-136 TDMA, andiDEN.

[0005] The above and other features of the invention including variousnovel details of construction and combinations of parts, and otheradvantages, will now be more particularly described with reference tothe accompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] In the accompanying drawings, reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale; emphasis has instead been placed upon illustratingthe principles of the invention. Of the drawings:

[0007]FIG. 1 is a block diagram showing the architecture of a typicalWLAN that supports Voice communications;

[0008]FIG. 2 is a timing diagram showing the initial mobile deviceregistration process;

[0009]FIG. 3 is a timing diagram showing the handoff procedure for amobile device with a PSTN-based call in progress;

[0010]FIG. 4 is a timing diagram showing the handoff procedure for amobile device with cellular network-based call in progress; and

[0011]FIG. 5 is a timing diagram showing the procedure for cellularnetwork-to-WLAN handoff.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] A typical WLAN is comprised of access points (APs) that areconnected to the enterprise LAN via an access controller (AC) or awireless LAN switch. The access controller/wireless LAN switch AC is thecenter of intelligence of the WLAN and is responsible for admissioncontrol, authentication and mobile device roaming coordination. OneAC/wireless LAN switch can control several APs and multiple ACs/wirelessLAN switches can be in the same network. Another device called theCellular Proxy (CP) is a gateway that connects the enterprise LAN to thecellular network. If the enterprise PBX is not VoIP-capable, thecellular proxy also provides the VoIP interface between the enterpriseLAN and the PBX. The Cellular Proxy is located in the part of theenterprise building that has a very good cellular coverage.

[0013] The Cellular Proxy is not necessarily tightly coupled to anycellular network. To any cellular network, it is a bank of radios. Ithides the details of the movement of the mobile devices within theenterprise premises from the cellular network thereby preventing thecellular network from making frequent updates to its database. Also, itcan connect to multiple cellular networks simultaneously because itcontains radios for different types of cellular network technologies,such as CDMA, AMPS TDMA, GSM TDMA, iDEN, WCDMA, CDMA2000, GPRS, 1XRTT,1xEVDO, and 1xEVDV. Thus, the Cellular Proxy can proxy for mobiledevices in CDMA-based and TDMA-based cellular networks simultaneously.

[0014] Mobile devices, such as laptops that are equipped with wirelessnetwork interface cards and personal digital assistants access thenetwork via the APs. A typical network is shown in FIG. 1. In thefigure, the access controller and/or wireless LAN switch are not shown;they are assumed to be part of the enterprise LAN infrastructure.

[0015] Each dual-mode mobile device MD has two telephone numbers: onenumber is assigned by the enterprise as an extension of the enterprise'sPBX, and the other number is assigned by the cellular carrier. Thedefault network for each mobile device is the enterprise (or hotspot)WLAN, which means that when a mobile device is turned on, it firstsearches for the enterprise's WLAN and registers with the network, if itis found. If the enterprise WLAN is not found the mobile device MD thenregisters with the cellular carrier network. The rationale for thisoperational requirement is to save the enterprise money by ensuring thatmobile device users do not accrue air charges when they are within thecorporate WLAN. All calls generated by the mobile device while it is onthe enterprise network are routed via the corporate PBX to the publicswitched telephone network (PSTN).

[0016] Since each mobile device has two telephone numbers, it can bereached in two ways: via the PBX when calls come from the PSTN, and viathe Cellular Proxy CP when calls come from the Cellular Proxy.Regardless of how the calls arrive, they are converted into voice overIP (VoIP) packets by the appropriate device and presented to the mobiledevice via the WLAN. These calls are based on the Session InitiationProtocol (SIP), which has the advantage over the ITU-T H.323 protocol inthat it is a lightweight protocol that leverages the Internet protocols.

[0017] Each mobile device MD goes through an initialization process whenit is turned on in the WLAN. As discussed earlier, the default networkis the enterprise WLAN. Therefore, when a mobile device is turned on itsends a Registration Request message to the appropriate authenticationserver in the enterprise LAN. After the authority server hasauthenticated the mobile device, it returns a Registration Completemessage to the device. The authentication server, which has informationon each mobile device's cellular service provider's network, then sendsa Registration Request message to the Cellular Proxy instructing thelatter to register the mobile device in the device's cellular carriernetwork. The Cellular Proxy CP first sets up a TCP connection to themobile device MD via the appropriate access controller (or wireless LANswitch) and access point before commencing the registration of thedevice in the cellular network. After the Cellular Proxy hassuccessfully registered the mobile device in the cellular network, itreturns a Registration Complete message to the authentication server.The Cellular Proxy then starts listening on the appropriate pagingchannel for calls destined for the mobile device from the cellularcarrier network and will deliver such calls to the mobile device via theappropriate access controller (or the wireless LAN switch) and accesspoint. The message flow for the registration process is illustrated inFIG. 2.

[0018] Consider a dual-mode mobile device that originates a call withina WLAN. As the user moves closer to the edge of the network the signalquality begins to degrade. The degradation will reach a point where thesignal strength is almost imperceptible, which causes the call to beterminated.

[0019] Here, the mobile device MD has the capability to monitor thesignal quality by measuring the signal-to-noise ratio (SNR). Assume alsothat from practical experience acquired through measurements it is knownthat when SNR reaches some threshold value d, the voice quality becomesunacceptable. The goal is to prevent the call quality from degrading tothis critical point. Thus, when the SNR drops to a cutoff value r>d, thesystem initiates a handoff with the objective of completing the handoffprocedure before the SNR drops down to the threshold value d. Thus thescheme operates in the following manner:

[0020] When a mobile device experiences SNR measurement value of r, itsends a Handoff Request message to the Cellular Proxy via the TCPconnection that exists between the two devices.

[0021] When Cellular Proxy receives the message it takes one of twoactions that depend on where the other party in the call is located.

[0022] If the other party is located in the PSTN, which means that thecall passes through the PBX, then it takes the following actions: 1) theCellular Proxy uses one of its own carrier-assigned telephone numbers tocall the mobile device's carrier-assigned telephone number; 2) since themobile device is not physically connected to the cellular network, theCellular Proxy will also receive the call on behalf of the device; 3)after receiving the parameters of the call from the cellular network,such as the channel or code to use, power level, etc., the CellularProxy will forward these parameters to the mobile device over the TCPconnection that it established between the two and commands the mobiledevice to switch its radio to the cellular network using thoseparameters; 4) the Cellular Proxy will then close the TCP connection,stop proxying for that device in the cellular network to avoid cloningproblems, and will thereafter forward the call to the mobile device overthe new connection established via the cellular network; and 5) onreceiving the call parameters, the mobile device will immediately switchits radio to the cellular network without having to register again sinceit has already been registered and authenticated in the cellularcontroller by the Cellular Proxy. As it moves from base station to basestation outside the enterprise network, the mobile device will besubject to the handoff mechanism that applies within the cellularnetwork.

[0023] This sequence of activities is illustrated in FIG. 3.

[0024] If the other party is located in the cellular carrier network,which means that the call passes through the Cellular Proxy withoutreaching the PBX, it takes the following set of actions: 1) since theCellular Proxy has all the parameters of the call, it returns theseparameters to the mobile device via the TCP connection between the twoand commands the mobile device to switch its radio to the cellularnetwork using those parameters; 2) the Cellular Proxy will then closethe TCP connection and stop proxying for the mobile device to avoidcreating cloning problems in the cellular network; 3) Upon receiving theinformation, the mobile device will tune its radio to the channelcurrently used by the Cellular Proxy and receive the call directly fromthe cellular network.

[0025] This sequence of activities is illustrated in FIG. 4.

[0026] Assume that the mobile device has a call in progress as it ismoved from the outside world into the enterprise premises. Theoperational rule is that if there is good cellular coverage within theenterprise premises, the call will be allowed to complete in thecellular network after which the mobile device will register in theenterprise LAN using the procedure described earlier. However, if thereis no good coverage within the enterprise premises, the device willinitiate a handoff with the Cellular Proxy. The procedure is as follows:

[0027] 1) if the SNR reaches the predefined cutoff value, the mobiledevice sends a short message service (SMS) message to the CellularProxy. The message contains information on the identity of the mobiledevice, such as its PBX extension and its cellular network telephonenumber as well as the parameters of the current call. The Cellular Proxymaintains a record of the cellular network that each enterprise mobiledevice is associated with. Therefore, with the information it receivedfrom the mobile device the Cellular Proxy sends a message to theauthentication server to expedite the authentication of the mobiledevice.

[0028] 2) The authentication server will provide emergency registrationfor the mobile device by broadcasting a Registration Invite message thatthe mobile device will respond to.

[0029] 3) After locating and authenticating the mobile device, theauthentication server forwards the device's location and networkconfiguration parameters like the IP address to the Cellular Proxy.

[0030] 4) After sending the emergency registration request to theauthentication server, the Cellular Proxy will start monitoring thechannel on which the mobile device was communicating and accumulatinginformation destined for the device until the device has beenauthenticated and registered in the network

[0031] 5) When the Cellular Proxy receives information on the device'slocation, it will set up a TCP connection to the device and forward allaccumulated packets to the device.

[0032] 6) After this, the operation becomes similar to that describedearlier. The Cellular Proxy listens on the channel and relaysinformation between the mobile device and the cellular network until theconversation is over and the connection is terminated. When the currentcall ends, the Cellular Proxy continues to listen on the cellularnetwork's paging channel for calls destined for the mobile device, asdescribed earlier.

[0033] This sequence of activities is illustrated in FIG. 5.

[0034] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method for communicating with a mobile devicecomprising emulating the mobile device on a wide area cellular wirelessnetwork using a fixed radio terminal.
 2. A method for handing off acommunication session with a mobile terminal between a local network anda wide area wireless network comprising: switching the communicationsession between a communication path passing from the wide area wirelessnetwork through a fixed radio terminal and over the local network to themobile terminal; and establishing a communication path passing from adevice on the local network to the mobile terminal not passing throughfixed radio terminal.
 3. An apparatus comprising: a local network,including a network access point for coupling a mobile device to thelocal network and a fixed radio terminal including an antenna forcommunicating on a wide area cellular network; a radio transceivercoupled to the radio antenna; and a network interface for communicatingover the local network.
 4. A method of deploying a gateway as aninterface between a local area network and multiple wide area cellularwireless networks that use different technologies simultaneously, themethod comprising: the gateway tuning multiple radios to differentcellular networks.
 5. A fast handoff scheme that does not require themobile device to register in the wide area cellular wireless network asit moves from the local area network into the wide area cellularwireless network in which the mobile device inherits the call parametersof the proxy device, which is the gateway, and switches its radio to thenew network without experiencing interruption in the call connection. 6.A method that permits migration to VoIP without the need for deployingexpensive VoIP switching infrastructure, comprising implementing theTDM-to-VoIP conversion in the gateway.
 7. A flexible method of routingcalls originating within an enterprise, comprising: forwarding the callsto the PSTN via the PBX; forwarding the calls to the cellular wirelessnetwork via the Cellular Proxy; and forwarding the calls to the Internetor a private IP network via the enterprise router using a virtualprivate network (VPN).
 8. A call control method in which the call istranslated by the gateway, which is referred to here as the CellularProxy, from a mobile device operating on a cellular network and a fixedwired telephone or user terminal connected to the PBX as a PBX phone orvia the LAN as a VoIP phone.
 9. A call control method in which anoutbound or inbound call enters or leaves the user organization premisesfrom the PSTN via a PBX or wireline carrier's Centrex service, or as aVoIP call via an private internet connection between two sites, as a VPNconnection between sites over the public Internet, or as a VoIP call toa public Internet or carrier's VoIP service.